In a known solid electrolytic capacitor, a sintered body is used as one electrode, a semiconductor layer working out to another electrode (counter electrode) is provided on the dielectric oxide film layer formed on the sintered body, and an electrically conducting layer is formed on the semiconductor layer. Such a solid electrolytic capacitor is demanded to have a high capacitance with a small size as much as possible. The sintered body is produced by appropriately shaping a valve-acting metal powder and sintering it. In the sintered body, pores are present and the portion other than pores forms a complicated shape such that a part of powder particles are melted and connected with each other.
As for the appearance capacitance of a solid electrolytic capacitor, 100% of capacitance is expected to appear when the dielectric oxide layer formed on the surface of a sintered body having the above-described internal shape is thoroughly covered up with another electrode. Assuming that the capacitance appearance factor is 100% when the other electrode is liquid and an electrolytic solution, it is demanded to approximate the capacitance appearance factor to 100% as much as possible when a solid semiconductor is used as the other electrode.
However, when the other electrode (counter electrode) is a semiconductor layer, the capacitance appearance factor can be hardly approximated to 100%. This is particularly difficult when the sintered body in the solid electrolytic capacitor has a large volume and moreover, a sintered body having a large CV value (a product of a voltage at electrochemical formation for forming a dielectric oxide film on the sintered body and a capacitance measured by using an electrolytic solution for the other electrode) is used.
On the other hand, JP-A-55-128816 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses that the ratio of pores of 2 μm or more occupying in the sintered body electrode is adjusted to 0.68 or more to decrease the density of a tantalum solid electrolytic capacitor element and thereby increase the electrostatic capacitance. However, this patent publication is silent on closed pores in the sintered body electrode. Also, there is not known a publication disclosing a sintered body electrode using, as the metal component, an earth-acid metal such as tantalum and niobium, where the pseudo-closed porosity is adjusted to 11% or less.